This application incorporates by reference Taiwan application Serial No. 090132623, filed Dec. 27, 2001.
1. Field of the Invention
The invention relates in general to a planar antenna, and more particularly to a planar inverted-F antenna of dual frequencies.
2. Description of the Related Art
Due to developments in communications technology, various wireless products are produced in great quantities. Recently, the Bluetooth system has been developed to enable communications between electronic products, such as computers, printers, digital cameras, refrigerators, TVs, air conditioners, and other wireless products. The frequency range of the ISM (Industrial Scientific Medical) band for Bluetooth is 2.4 to 2.4835 GHz. If more and more wireless products employ the Bluetooth system, the single frequency band of the ISM will not sufficiently support the large volume and transmission rate. The same situation also happens in the other wireless communication systems of ISM 2.4 GHz, such as WLAN (wireless local area network) and HomeRF (Home radio frequency).
Therefore, a dual-frequency antenna has been developed to reduce the volume of the wireless communication products by combining two frequencies in an antenna. Furthermore, the product of a dual-frequency antenna will be more competitive if the size of the dual-frequency antenna is minimized. Accordingly, a PIFA (planar inverted-F antenna) is developed to decrease the amount of space occupied, wherein the length of the PIFA is reduced to xcex/4, instead of xcex/2, which is the length of the traditional planar antenna. This reduction in the size of the planar antenna makes it possible to be concealed within most of the present-day communication devices.
Please refer to FIG. 1, which shows the structure of a PIFA (planar inverted-F antenna) according to a traditional design. The PIFA 100 is composed of a radiator 110, a grounding plane 130, a medium 150, a shorting pin 170, and a feeding means 190. The medium 150 is used to separate the radiator 110 and the grounding plane 130, and is positioned between the two. The material of medium 150 can be air, dielectric substrate, or the combination of them. The radiator 110 is coupled to the grounding plane 130 by the shorting pin 170, which is made of metal. The feeding means 190, such as SMA connector, can be equipped on the ground and coupled to the radiator 110 to deliver the microwave signal. The radiator 110 and the grounding plane 130 are made of metal, wherein the radiator 110 can be of various patterns, according to the different requirements.
Basically, the structures of each PIFA are the same, for instance, the separation of the grounding plane and the radiator by the medium, the coupling of the radiator to the grounding plane by the shorting pin, and the coupling of the feeding means 190 to the radiator. The operational characteristic of the PIFA is determined by the pattern of the radiator. Shown in FIG. 2A is the radiator pattern of a PIFA with dual frequencies, according to a traditional design. The grounding point 271 and the feeding point 291 are, respectively, the parts of the shorting pin contacting with the radiator 210A and the feeding means contacting with the radiator 210A, wherein the former is represented by a square and the latter is represented by a circle. The same representations for the grounding point and the feeding point are used in the following figures.
In FIG. 2A, an L-shaped slit is embedded in the radiator 210A, wherein two surface current paths of L1 and L2 for the dual frequencies are formed. The radiator 210A resonates at the higher frequency, such as 5.8 GHz, with the shorter path L1, and resonates at the lower frequency, for instance 2.4 GHz, with the longer path L2.
Please refer to FIG. 2B, which shows a PIFA of dual frequencies according to another traditional design. As the radiator 210B is excited, the U-shaped slot is responsible for the formation of two current paths in the radiator 210B, wherein the shorter current path L1 produces the higher frequency and the longer current path L2 produces the lower frequency.
The detailed configurations of the PIFAs in FIG. 2A and FIG. 2B are disclosed in xe2x80x9cNew slot configurations for dual-band planar inverted-F antennaxe2x80x9d, Microwave Optical Technology Letters, vol. 28, No. 5, Mar. 5, 2001, pp. 293-298. Such kinds of dual-band PIFA usually cannot afford a sufficiently broad bandwidth. In U.S. Pat. No. 5,764,190, the inverter-F antenna is designed using the capacitive effect or a capacitive feed, which can provide an adequate bandwidth. However, this design is relatively very complicated and the fabrication cost is very high.
To solve the problems mentioned above, the present invention discloses a PIFA with broad bandwidth, simple structure, and low cost.
It is therefore an object of the invention to provide a dual-frequency PIFA with the advantages of broad bandwidth and simple structure.
In accordance with the object of the invention, a dual-frequency PIFA is disclosed, wherein the said PIFA has a first operational band, such as 2.4 GHz ISM band, and a second operational band, such as 5.8 GHz ISM band. The dual frequency PIFA comprises a grounding plane, a main radiating device, a parasitic radiating device, a medium, two shorting pins and a feeding means, wherein the main radiating device and the parasitic radiating device are coupled to the grounding plane through shorting pins, respectively. The feeding means positioned on the grounding plane is coupled to the main radiating device for transferring the microwave signal. The excitation of the main radiating device triggers the excitation of the parasitic radiating device by the coupling of the electromagnetic energy. The first resonance mode of the main radiating device enables the PIFA to operate in the first operational band and the first resonance mode of the parasitic radiating device enables the PIFA to operate in the second operational band. Thus, the PIFA can operate in dual frequencies.
Please note that the structure of the present invention is not limited to the PIFA. It is also applicable in a planar antenna.